Am 17.10.2012 06:16, schrieb Eric Blake:
I'm still seeing the corner case of:
qemu-kvm -add-fd fd=3,set=1 -add-fd fd=4,set=2 4<&-
where the dup(3) will populate fd 4 prior to the point where we get to process the -add-fd fd=4 command to notice that the user started qemu-kvm with fd 4 closed, and thus qemu will silently proceed to use the wrong fd.
On the other hand, I'm not sure if that corner case is worth worrying about, or if we just chalk it up to user stupidity (aka libvirt programmer stupidity) if they did something like that (most likely, because the management app forgot to clear FD_CLOEXEC before exec()ing qemu-kvm).
If you specify an FD number that isn't actually open when qemu is stared, you can get any FD that qemu opens internally. I think the correct answer to this problem is "then don't do that".
Hmm, this makes me wonder if I can do something crazy like:
qemu-kvm -add-fd fd=4,set=1 -qmp /dev/fdset/1
to open a monitor on the fd I just passed in?
I think so. At least on my side it was intended to allow this.
And what if so, what then happens on that monitor if I request that fdset 1 be removed?
The same as with block devices: The fd stays open until the monitor connection is closed. A closed monitor also triggers fd garbage collection, so at this point the original fd would be closed (well, assuming that you had only one monitor). Kevin